It is commonly known that metals, both in the liquid and solid state, when cooled undergo a decrease in volume, a so-called thermal contraction. In moulds, in which there is a non-uniform distribution of heat quantity in the mould cavity after the pouring, and in which for this reason not all the parts of the castings solidify at the same time, this causes the region of the casting solidifying last to give off liquid metal to compensate for the contraction in the regions of the casting having solidified earlier, this leading to flaws in the casting commonly called "shrinkages", appearing either as depressions in the surface of the casting or as hollows (cavities or micro-shrinkages) within the casting. In order to avoid these faults, the skilled person can take a series of steps, of which the most common is the use of after-feeding reservoirs, i.e. cavities in the mould being filled with metal during the pouring operation and having such dimensions that the metal in them solidifies later than the last-solidifying regions of the casting, being connected to these regions through ducts with a relatively large cross-sectional area so as to enable them to after-feed these regions with liquid metal during the solidification of the casting.
Such after-feeding reservoirs are mainly known in two types, viz. as feeders or risers, i.e. substantially cylindrical cavities extending from the duct connecting them to the casting to the upper surface of the mould, and as internal or enclosed cavities in the mould, so-called "suction buds", placed in the immediate vicinity of the region of the casting to be fed.
Relative to the latter type, the former type exhibits the advantage that the higher metallostatic pressure at the after-feeding location, i.e. the pressure of the overlying metal column, to a high degree supports the after-feeding by pressing the feeding metal to the connecting duct into the casting, while in the latter type, the pressure diminishes during the after-feeding process.
On the other hand, the latter type exhibits the advantage of normally giving a higher metal yield in the casting process, i.e. a lesser quantity of metal to be separated from the castings for subsequent re-melting (recycling), this also reducing the energy consumed for melting.
Compared to moulds with a horizontal parting surface, the top surface of moulds with a vertical parting surface has a relatively small surface area, and for this reason, the latter type of moulds allows only to a low degree the use of feeders or risers for after-feeding purposes, and thus, for this purpose it is generally necessary to use the above-mentioned "suctions buds" with the associated disadvantage mentioned above, i.e. the relatively lower metallostatic pressure for pressing the after-feeding metal in through the duct to the casting. This disadvantage is even more noticeable when after-feeding light-metal castings, i.e. castings of aluminium and its alloys or magnesium and its alloys, due to the relatively low specific weight of these metals.
Casting of light-metal castings in moulds with vertical parting surfaces is of commercial interest especially in two cases, viz. by casting in permanent moulds, e.g. pressure die-casting, and by casting in moulds in a string-moulding plant, such as DISAMATIC.RTM., a string-mould-making plant manufactured and marketed by the Applicants. With such light-metal alloys it occurs frequently that the ingate system and especially the after-feeding reservoir after the solidification constitute roughly one-half of the weight of the casting, requiring separation from the casting proper and possibly recirculation, causing extra work and a large energy loss when superfluous material is first melted and then solidifies.
Because of the problems mentioned above, it is known when carrying out casting processes of this kind to reduce the excess cast material from the ingate system and the after-feeding reservoir by applying a pressure, e.g. in the form of a gas pressure, to the after-feeding reservoir in order to press the molten metal into the mould cavity to compensate for the contraction of the parts having solidified. The equipment for such casting processes are known e.g. from PCT application WO 95/18689. There are mainly two types of equipment of this known kind, that is capable of applying pressure to an after-feeding reservoir during movement of the mould.
One of these types is constituted by complicated individual units adapted to be attached to or integrated in the mould and are capable of supplying pressure independently; these units are complicated and costly, and they may even make it difficult to manufacture the moulds.
In contrast, the other type is constituted by coupling elements adapted to be integrated in the moulds during the manufacture of the latter, subsequently being supplied with pressure by means of complicated pressure-transfer equipment that is relatively costly and can complicate the construction of pouring channels or make it necessary to alter the latter.
In practice, this known equipment has functioned satisfactorily and has made it possible to reduce the size of the after-feeding reservoir, thus reducing the energy loss by first melting the material and then remove it from the castings, at the same time as the quality of the castings is the same or even improved, due to the after-feeding reservoir being supplied with pressure all the time during the solidification of the casting.
Even though so far, this equipment has been functioning very well, it does suffer from the disadvantage of consisting of relatively complicated units to be integrated into the moulds, comprising either complicated extra equipment for each mould to apply pressure to the after-feeding reservoir, or complicated pressure-transfer equipment, or else having required a special arrangement and construction of the casting and cooling sections, the latter being costly and possibly setting limits to the construction of the moulds, because they are to be supplied with pressure from the equipment in the casting/cooling section. Thus, already during the work of constructing the moulds, it has been necessary to take into consideration that not only the after-feeding reservoir, but also pressure-transfer elements were to be integrated in the moulds when manufacturing the latter.